1. Field of the Invention
The invention relates to an ultraviolet light-emitting material, which emits ultraviolet light, in particular UV-C having a wavelength of less than 280 nm, and an ultraviolet light source manufactured by using the same ultraviolet light-emitting material.
2. Background of the Invention
Once ultraviolet is absorbed into bacterial cell, a nuclear protein configuration of the cell is chemically modified and then the vitality and metabolism of the bacteria is adversely affected. As a result, the proliferative ability of the bacteria is lost, and plasma breakage occurs, thereby causing bacterial death. Such ability (i.e., bactericidal capability) of the ultraviolet is maximized at near ultraviolet wavelength of 254 nm, which corresponds to the area of UV-C. Furthermore, the afore-mentioned bactericidal capability of the ultraviolet can be used for a wide variety of applications including, for example, a bactericidal light source for a hospital, a food processing system, or water supply and sewerage systems, and a light source for a degradative treatment of environmental pollutant by use of light catalyst.
While a bactericidal device employing a low pressure mercury lamp (peak wavelength: 254 nm) has been usually used as a near ultraviolet light source for emitting near ultraviolet ray, the use of mercury has been strictly regulated under RoHS Regulations and the like due to its adverse effect on environment or a human body in the case of leakage.
For the above reasons, ultraviolet light-emitting material employing hexagonal boron nitride (hBN) as described in, for example, JP 2007-9095 A has been investigated and developed as a new light-emitting source. Such new light-emitting source has been considered to be a substitute for mercury used with a low pressure mercury lamp.
In the followings, the low pressure lamp and CL (cathode luminescence)-type ultraviolet light source employing hBN as described in JP 2007-9095 A as a light-emitting source were compared with respect to the wavelength of output spectrum, bactericidal performance, and operating life.
A. Wavelength of Output Spectrum
FIG. 7A is a graph showing the comparison between the low pressure mercury lamp and the afore-mentioned hBN ultraviolet light source with respect to ultraviolet light spectrum and wavelength-dependency of bacteriocidal effect.
As shown in FIG. 7A, the output spectrum peak wavelength of the low pressure mercury lamp is 254 nm, which corresponds to the output spectrum of the sterilization ray, and the output spectrum peak wavelength of hBN ultraviolet light source is 223 mm, which is shifted to a short wavelength as much as approximately 30 nm in comparison to the low pressure mercury lamp. Furthermore, when comparing each ultraviolet spectrum peak wavelength of the above two light sources with the relative bacteriocidal effect value as defined by JIS-Z-8811, hBN ultraviolet light source has only very limited wavelength adapted for bacteriocidal use (i.e., sterilization), thereby being hardly capable of obtaining such bacteriocidal effect as can be obtained by the low pressure mercury lamp.
B. Bacteriocidal Performance Comparison
FIG. 7(B) shows a comparison between the low pressure mercury lamp and hBN ultraviolet light source with respect to luminescence intensity and bacteriocidal capability.
As shown in FIG. 7(B), the low pressure mercury lamp has a luminescence intensity of 10.0 mW/cm2, and hBN ultraviolet light source has a luminescence intensity of 0.9 mW/cm2. In view of the above, the luminescence intensity of hBN ultraviolet light source is only one tenth of that of the low pressure mercury lamp.
Bacteriocidal efficiency and bacteriocidal capability for the low pressure mercury lamp and hBN ultraviolet light source were respectively calculated in following steps:
Step 1: calculate a normalized spectrum f(λ) such that the overall integral value of the luminescence spectrum is 1 (i.e., ∫f(λ)dλ=1);
Step 2: Determine bacteriocidal efficiency as ∫X(λ)f(λ)dλ given that relative bacteriocidal effect value is X(λ).
Step 3: Determine bacteriocidal capability by multiplying the luminescence intensity by the determined bacteriocidal efficiency.
By comparing the bacteriocidal efficiencies and bacteriocidal capabilities as determined in accordance with the above steps between the above two light sources, it has been confirmed that the low pressure mercury lamp is superior to hBN ultraviolet light source in terms of bacteriocidal efficiency and bacteriocidal capability.
As mentioned previously, in case of using hBN ultraviolet light source, any material such as mercury which can adversely affect human bodies is not used, thereby realizing environmentally-friendly bacteriocidal method, as well as, due to surface emitting making the area of light-emitting face larger. However, hBN ultraviolet light source is inferior to the low pressure mercury lamp in terms of luminescence intensity, ultraviolet spectrum peak wavelength, and operating life.
The inventors have continuously investigated a substitute for the above two ultraviolet light-emitting material, which can overcome the drawbacks of the above two types of ultraviolet light-emitting material while maintaining the advantages of the above two types of ultraviolet light-emitting material. Surprisingly, the inventors have found a new ultraviolet light-emitting material, which has luminescence intensity and a wavelength near the peak wavelength of the low pressure mercury lamp and in which due to surface emitting as performed by hBN ultraviolet light source luminescence efficiency can be enhanced, and a new ultraviolet light source employing the same ultraviolet light-emitting material.